Reciprocating pumps such as pumpjacks are typically operated with a fixed motor speed during a revolution of the crank arm. The speed, acceleration and position of the linear motion applied to the rod string at the horsehead are determined by the speed, acceleration and position of the crank arm and the geometry of the pumpjack. The geometry of a typical pumpjack is depicted in FIG. 1. Conventional operation of a pumpjack is to maintain a constant crank speed. As a result, the geometry of the pumpjack dictates a rod speed which follows a curve which is sinusoidal in nature.
Adjustments to optimize well production have historically involved changing the geometry of the pump or by increasing or decreasing the overall rotational velocity of the crank. Within a cycle, crank speed typically remains fixed and the dynamics of the pump are determined by the geometry.
Methods have been implemented where the speed has been varied within the stroke to generally increase the speed during the upstroke to maximize efficiency and decrease the speed on the downstroke to eliminate pounding against fluid columns. For example, in U.S. Pat. No. 4,102,394, a control system for a variable speed electric motor used to power a pumpjack is disclosed. The control system is said to allow for greater upstroke speed versus downstroke speed and to vary the stroke frequency in response to oil levels in the well and in storage facilities. However, no detailed disclosure of the control system is provided. As well, the system does not allow for customized speed profiles to be implemented.
Therefore, there is a need in the art for a control system, including methods and apparatuses, for allowing convenient and complete control of crank speed and rod velocity within a stroke cycle.